The present invention pertains to a tandem brake power booster for motor vehicles with two booster units comprising a vacuum chamber and a working chamber each, which are arranged in a housing and are separated by a partition, wherein the pressure difference between the pressure in the vacuum chamber and the pressure that can be influenced by a switching device in the corresponding working chamber acts in each booster unit on a piston separating the vacuum chamber and the working chamber and the forces which act on the pistons and are proportional to the pressure difference are bundled via mechanical intermediate members with the forces applied to a corresponding brake pedal by a vehicle driver by means of at least one transmission element each.
Brake power boosters of the above-described type of this class are used in the automobile industry in motor vehicles wherever high brake powers are to be obtained with weak pedal forces. Tandem arrangements are used where an enlargement of the piston diameter of a brake power booster provided with only one booster unit is not possible because of space limitations. Tandem brake power boosters with two booster units arranged one behind the other achieve an increase in the brake power by about 20% at a housing external diameter of 200 mm compared with a single brake power booster with a housing external diameter of 250 mm, and it should be borne in mind that the brake power boosters operated, in principle, with the vacuum of the engine of the motor vehicle bring about, in general, a boosting of the brake pedal forces by a factor of up to 5.
The boosting of the brake power takes place in the tandem brake power boosters known from the prior art, in general, during each braking operation, even when a great increase in the brake power is not necessary for the intended braking operation at all. This property of prior-art brake power boosters is especially highly pronounced in tandem brake power boosters and it adversely affects the ability of the braking power to be metered.
The object of the present invention is therefore to further improve a tandem brake power booster of this class such that the power boosting, which can be achieved by the tandem brake power booster, is made available only when a great braking deceleration is necessary.
This object is accomplished according to the present invention by the switching device having a first on-off valve actuated directly by the corresponding brake pedal for admitting pressure to the working chamber of one of the two booster units and a second on-off valve controllable as a function of the pedal power applied to the brake pedal and the speed with which the pedal is actuated for admitting pressure to the working chamber of the second booster unit.
Admission of pressure to the second working chamber independently from the working chamber of the first booster unit is brought about by this solution.
The pedal power acting on the brake pedal and additionally the speed with which the pedal is actuated are decisive criteria for when the driver of the vehicle wishes to perform a normal braking or an emergency or panic braking. Depending on these two values, which can be determined by means of sensors at the brake pedal, it is thus possible to control whether only one booster unit is needed for a braking operation or whether correspondingly higher power, which is necessary for an emergency or panic braking, is to be applied to the brake system.
In addition to the first booster unit, pressure is also admitted in the second case to the additional booster unit acting independently from the first booster unit through the connected on-off valve. Thus, sufficiently high actuating forces are available for the braking operation in cases in which they are necessary; on the other hand, the ability of the brake system to meter the power is not affected adversely during normal operations. The separate actuation of the additional booster unit as a function of the pedal power and the speed with which the pedal is actuated can also be used if the corresponding motor vehicle has an electronic brake assist. This technical means is an electronic control unit which additionally recognizes whether the driver of the vehicle again reduces the pedal power during an emergency braking because of special circumstances and thus brings about an unintended increase in the braking distance. In such a case, the recognition and the evaluation of the braking operation by the brake assist can also contribute to the supply of the necessary brake power due to the design of the tandem brake power booster according to the present invention.
A particularly inexpensive and reliable design of the second on-off valve controllable as a function of the pedal power applied to the brake pedal and the speed with which the pedal is actuated for admitting pressure to the working chamber of the additional booster unit is obtained by this valve being actuated electromagnetically.
An exemplary embodiment of the subject of the present invention will be explained below in greater detail on the basis of the drawing attached.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.